In the fabrication of semiconductor devices, plasma chambers commonly are used to perform various fabrication processes such as etching, chemical vapor deposition (CVD), and sputtering. Generally, a vacuum pump maintains a very low pressure within the chamber while a mixture of process gases continuously flows into thc chamber and an electrical power source excites the gases into a plasma state. The constituents of the process gas mixture are chosen to effect the desired fabrication process.
In essentially all etching and CVD processes, and in many sputtering processes, the semiconductor wafer or other workpiece is mounted on a cathode electrode, and a radio frequency (RF) electrical power supply is connected, through a DC blocking capacitor, between the cathode electrode and an anode electrode in the chamber. Most commonly, the walls of the chamber are metal and are connected to the RF power supply to function as the anode electrode. When the chamber walls are the anode, they typically are connected to electrical ground.
The body of the plasma has a positive charge such that its average DC voltage is positive relative to the cathode and anode electrodes. Because the RF power supply is connected to the cathode and anode electrodes through a DC blocking capacitor, the respective DC voltages at the cathode and anode can be unequal. Specifically, because the cathode's surface area facing the plasma is much smaller than the anode's surface area facing the plasma, the cathode is much more negative than the anode. In other words, the voltage drop between the plasma body and the cathode is much greater than the voltage drop between the plasma body and anode. This voltage asymmetry is a widely observed phenomenon, although its physical cause is complex and not completely understood. (See M. A. Lieberman et al., "Principles of Plasma Discharges and Materials Processing," pub. John Wiley & Sons, 1994, pages 368-372.) The negative DC voltage at the cathode relative to the anode commonly is referred to as the "cathode DC bias".
The negative DC bias voltage at the cathode accelerates ions from the plasma to bombard the semiconductor wafer with a kinetic energy approximately equal to the voltage drop between the cathode and the plasma body. The kinetic energy of the bombarding ions can be beneficial in promoting the chemical or physical reactions desired for the semiconductor fabrication process.
However, bombarding ions having excessive kinetic energy can damage the device structures being fabricated on the semiconductor wafer. Therefore, it often is desirable to reduce the cathode DC bias.
A known method of reducing the cathode DC bias is to reduce the level of RF power applied to the cathode electrode. However, reducing the RF power undesirably reduces the rate of dissociation of molecules in the plasma, thereby undesirably reducing the rate at which the fabrication process is carried out (i.e., increasing the time required to fabricate a semiconductor device). Therefore, a need exists for an apparatus and method for reducing the cathode DC bias other than by reducing the RF power supplied to the cathode.
Certain semiconductor fabrication processes require more highly energetic ion bombardment than other processes. It is desirable for a single plasma chamber to be adaptable to a number of different processes. Therefore, a need exists for a method of adjusting the cathode DC bias in a given chamber other than by adjusting the RF power supplied to the cathode.